This invention relates to a method for preventing the mixing of fluids in a riser of the type used in offshore petroleum production. The invention also includes a device for carrying out the method.
In offshore petroleum production, drilling vessels, as shown in FIG. 1, are used on the sea surface to drill wells in the seabed. A wellhead is installed on the seabed and is sealingly connected to the well casing and provided with the necessary valve assembly (BOP) and connecting devices to a riser which connects the wellhead to the drilling vessel at the sea surface. The riser is used in connection with the return and circulation of drilling fluids from the well and is normally filled with drilling fluid in the drilling phase. The riser is in addition equipped with longitudinal, externally located smaller pipes (choke, kill and optionally booster line) which are connected to a valve manifold on the drilling vessel. These pipes are used, inter alia, in connection with the replacement of fluid in the riser and for replacing drilling fluid in the well if the well pressure rises and it is necessary to close the valve assembly (BOP) that is connected sealingly to the wellhead underneath and the riser above in order to prevent undesirable outflow of drilling fluid.
During drilling a drill string is used, inserted into the riser and the casing as shown in FIG. 2, which may consist of sections of pipe and which at its lower end has drill collars and a drill bit. The drilling fluid is pumped under high pressure down through the drill string and supplies the drill bit with hydraulic effect and thereafter transports the drilled cuttings back to the drilling vessel via the annulus between the borehole and the drill string, then in the annulus of the section with casings and then in the annulus between the riser and the drill string. During drilling it is sometimes necessary to disconnect the riser from the wellhead and valve assembly, for example, because of bad weather or relocation of the drilling vessel. Before the riser is disconnected, the wellhead valve assembly (BOP) must be closed and the drilling fluid that is inside the riser must be replaced by water so as to prevent the release of drilling fluid into the external environment during the disconnect operation.
When disconnecting the riser because of bad weather during drilling, the pipe portion that is below the wellhead will usually have a length that makes it impractical to pull up the entire length. In such cases, the pipe portion that is between the drilling vessel at the sea surface and the wellhead on the seabed is pulled up onto the drilling vessel and has a hang-off tool attached thereto in order to then run this tool back down to the wellhead where it is hung off and disconnected from the pipe portion that is above the wellhead. After this, the pipe portion is normally pulled up onto the drilling vessel and the riser can be disconnected after the drilling fluid has been replaced with water.
The purpose of replacing the drilling fluid in the riser is to keep the drilling fluid for later use and to prevent it from escaping into the external environment during the disconnect operation. Usually seawater is used, which is pumped down to the wellhead via a valve manifold, through the externally located smaller pipes (kill, choke and booster line) and displaces the drilling fluid out of the riser and back to the drilling vessel's storage tanks. It is a problem that the water used to displace the drilling fluid mixes with the drilling fluid and contaminates it. A portion of the drilling fluid must therefore, in order to be further usable, undergo a costly purification process or it must be destroyed.
NO 313712 describes a method and a system for separating water and drilling fluid. According to this document, a sealing body is fastened to the upper part of the drill string, above the hang-off tool, when the drill string is pulled up in order to attach the hang-off tool. The sealing body, together with the drill string to which it is fastened, is moved vertically in the riser by either the weight of the drill string or the vessel's drawworks and thereby displaces the fluid that is to be replaced. The sealing body is intended to seal against the inner wall of the riser, prevent the mixing of fluids and be moved in the longitudinal direction of the riser in that it is connected as a fixed element of the drill string, above the hang-off tool, and displaces the fluid that is to be replaced.
There are several problems that may arise when using such a method and device. When the sealing body is moved downwards in the longitudinal direction of the riser and the fluid inside the riser is displaced up into the smaller pipes (choke and kill pipes), pumping and varying pressure will arise as a result of the vessel's heave. When the rate of movement increases, the pressure against the sealing body and the elastic sealing elements will increase and be subjected to loads which may result in leakage and mixing of fluids in the riser. The smaller pipes have a small internal diameter and require a great deal of energy to have fluids transported at high speed through the pipes. It is the drilling vessel's high-pressure pumps that are used for this purpose. This makes it difficult for the sealing body, together with the drill string, to supply sufficient energy for the displacement of the fluid inside the riser as they do not have sufficient mass.
The weight of the drill string is often also not sufficient to provide satisfactory pressure from the sealing body and a satisfactory rate of movement in the longitudinal direction of the riser. The reason is that the smaller external pipes have a small diameter and large flow resistance. In addition, flow resistance in the choke and kill manifold must be overcome. Furthermore, it may be a problem that the inner walls of the riser are not prepared for sealing off high differential pressure across the sealing body. The hoisting of the drill string results in the occurrence over time of tears in the surface which make sealing against a movable sealing body with high differential pressure difficult, and an optional scraping of the pipe wall prior to use of the sealing body will mean an additional running of the drill string in the riser, which is costly and time-consuming. When the drill string and the sealing body are hoisted up internally in the drilling vessel's riser, the fluid that is to be replaced runs out into a suitable place on the drilling vessel and fresh fluid is filled through the externally located smaller pipes using the vessel's high-pressure mud pumps. As the drill string is pulled out of the riser, drill pipes must be disconnected from the drill string. The drill string is stopped and hung off on slips on the drilling vessel, which take up the weight of the drill string. The high-pressure mud pumps must be stopped before the drill string is hung off in the slips so as to prevent the sealing body from being moved further upwards in the riser because the force of the pressure below the sealing body may exceed the drill string's mass. The vertical heave of the drilling vessel may also result in the same in that the sealing body is moved downwards in the riser but is stopped because the fluid that is below the sealing body and in the externally located smaller pipes cannot be returned quickly enough in relation to the vertical heave rate of the drilling vessel. Thus, there are a number of disadvantages and potential problems with the prior art as described in NO 313712.
The aim of the present invention is to be able to remedy the disadvantages of the prior art as described above. One of the objects is to provide a method and a device which prevent the mixing of fluids in connection with the emptying of drilling fluid in a riser when the riser is disconnected from a wellhead. It is also an object to provide a device which eliminates the disadvantages, in the event that there are internal deposits in the riser, of having to run a scraper in a separate operation before the insertion of a fluid separating body. It is a further object to provide a solution which better utilises existing pipes in order to replace fluid in a riser. It is also an object to provide a solution that can be arranged at the point of use and quickly actuated or taken up again without being used.
The objects are achieved according to the invention by the features that are disclosed with regard to the method and the device as disclosed in the description below and the attached claims.
The invention relates to a method for replacing fluid in a riser of the type used in offshore petroleum production, where the riser constitutes a connection between a wellhead on or above the seabed and a drilling vessel as explained above. According to the invention, a body in a first state is run down to a point of use in the riser, where it is actuated into an active sealing state against the riser in which it is designed to segregate the fluids, after which the body is moved mechanically or hydraulically in the longitudinal direction of the riser and replacement of fluid is accomplished without any mixing of the fluids.
According to a preferred embodiment of the invention, the body in the first state is run down to the point of use, this point of use preferably being in a position immediately above the wellhead. In one embodiment such a position will be above the wellhead and the opening of external smaller pipes between the riser and the vessel, for example, kill lines. The conveying of the body is carried out in a suitable manner preferably by using a conveying means such as a drill pipe, wireline, coil tubing, ceramic cable or the like. According to the preferred embodiment, the body is released from the conveying means at the point of use and is actuated into an active sealing state against the riser, so that it acts as a separating body between fluids in the riser. The body may also be run down to the point of use without using a mechanical conveying means, for example, by being pumped down to the point of use. In such a case, the body could be in an active state when placed in the riser, in such instance there being no drill pipe, wireline or other means inside the riser.
In a preferred embodiment, the body, when run down to the point of use in the riser, is arranged in an installation device. At the point of use, the body will be released from the installation device and actuated into the actively sealing state against the riser. The installation device may be a-suitable device that is connected to the conveying means and released from the conveying means at the point of use before the body is released into an active state. The installation device need not be conveyed by a conveying means. The installation device can, together with the body, be run down to the point of use by pumping, after which the body is released into an actively sealing state against the riser. The installation device may also be connected to the conveying means and accompany it when the conveying means is pulled up from the riser after the body has been released, or it may form a part of the hang-off tool and thus first be pulled up together with the hang-off tool. In such a case as the latter, the release of the hang-off tool may coincide with the actuation of the actively sealing body against the riser. According to one aspect of the invention, the body can be released and actuated into a sealing function against the riser by means of a positioning device in cooperation with the installation device.
According to another embodiment of the invention, the body in the first state may be run down through a drill pipe to the point of use in the riser where, upon release from the drill pipe, it is actuated into a sealing state against the riser. In this embodiment, the body, also in the first state when run through the drill pipe, may be in sealing abutment against the interior of the drill pipe and thus form a barrier between fluids.
In one embodiment, a scraping means is attached to the conveying means and/or the installation device which, upon introduction of the body to the point of use, performs a cleaning of the riser wall. Thus, the riser is cleaned before the body, which in its active state is sealing against the riser, is to be run into the riser whilst it has a sealing function between two fluids in the riser.
According to one embodiment of the method, fluid is pumped into the riser through one or more pipes connected to the riser immediately above the wellhead, said fluid thus displacing the body upwards in the longitudinal direction of the riser.
The invention also relates to a device for use to separate fluids in a riser of the type used in offshore petroleum production, where the riser constitutes a connection between the wellhead on or above the seabed and a drilling vessel. According to the invention, the device comprises a body which has a first state in which it is conveyed to a point of use in the riser and a second state in which it has a sealing function against the riser, wherein it is sealingly in abutment against the riser and separates fluids in the riser.
According to a preferred embodiment of the invention, the body in the first state is arranged in an installation device which allows fluid to flow through the installation device, comprising devices for releasing the body at the point of use. The installation device may be constructed with an ejecting mechanism and/or a positioning mechanism which is configured in a suitable manner, such as, but not limited by, hydraulically/mechanically compressed, spring function, cylinders, electrical, gas expansion etc., in order to release the body from the installation device.
Furthermore, the installation device may comprise means for releasable connection to a conveying means, for example, a drill pipe, coil tubing, wireline etc. In another embodiment, the installation device may alternatively be connected to or form a part of a hang-off device for hanging off a lower portion of the drill pipe at the wellhead when the lower portion of the drill pipe is released from an upper portion of the drill pipe that is in the riser.
In one embodiment, the installation device may comprise scraping devices for scraping the inner wall of the riser when the installation device and the body are run to the point of use for the body in the riser. Alternatively, it is conceivable that the installation device may comprise other devices for carrying out other activities in the riser when the body is run into the position at which it is to be actuated into a sealing state against the riser.
In one possible embodiment, the installation device may comprise a sleeve which has an outer diameter smaller than an inner diameter of the riser, wherein in the annular space between the outer surface of the sleeve and the riser there may be provided scraper elements which allow fluid to flow along the sleeve and where the body is releasably arranged inside the sleeve. The installation device may be constructed with a release mechanism for the separating device and openings for the passage of fluids.
By means of a method and device according to the invention it is possible to separate fluids in a riser during the replacement of, for example, drilling mud, without the fluids being mixed, whilst eliminating or reducing the drawbacks of the prior art.